In the automotive industry, the formed plastic parts (fascia) used are often painted. This is to provide a smooth, oftentimes metal-like finish. The paint covers molding and other substrate imperfections. Decorative effects such as mirror finishes, texturing and color match of differently processed parts are also achieved by painting.
Examples of plastics that are paintable and used in the automotive industry include polycarbonate/polybutylene terephthalate, polyphenyleneoxide/nylon 66, polyphenyleneoxide polybutylene terephthalate, polyether-polyester copolymer/polybutylene terephthalate, polycarbonate (both unfilled and glass filled), polyphenyleneoxide (both unfilled and filled) and a resin/glass material.
Most plastics need to be cleaned before painting them. The most common contaminants that must be removed before painting are mold release agents, shop soil, material handling soil, machining soils, fingerprints and plastic sanding dust. These contaminants are removed by chemical means such as detergents.
The process of cleaning plastic parts generally entails a wash stage, a rinse stage and a final rinse stage with deionized water. The plastic part can be either spray washed or immersed in the detergent. The most common processes are either an acidic detergent wash stage followed by multiple rinse stages, or an alkaline detergent stage, followed by a rinse stage, an acidic detergent wash stage, and multiple rinse stages. One process, for example, employs in stage 1 an alkaline wash, followed by a tap water rinse in stage 2. Stage 3 is an acidic wash with a tap water rinse in stage 4. Stages 5 and 6 can then be deionized water rinses.
When the plastic part is to be painted, the cleaning operation is performed in a production line with the freshly cleaned part sequentially conveyed into the paint booth. Problems can occur however, when the freshly cleaned part is not fully dry. This can lead to "spotting" when painted and will result in an imperfect paint job. This will require stripping of the part and/or an additional painting procedure.
Further, when a bead of water dries on the plastic surface, ionic salts such as Cl, Ca, Mg and K will remain on the surface of the plastic. After the plastic has been painted, there will be a visual defect there. Exposure to humidity which can cause increased water sensitivity can lead to blistering of the paint. Adhesion problems could also possibly result.
Removing the aqueous rinse water from the surface of the plastic requires a drainage enhancing solution which will effectively aid in drying the plastic part to be painted. The present inventors have discovered an aqueous solution which will aid in reducing spotting on the external surface of plastic parts. More surprisingly, the present inventors have discovered a drainage enhancing solution that will not be rinsed off the surface of the plastic parts by subsequent rinses.